It is known to use sample-and-hold devices or circuits at the input of an analog-digital converter, to acquire a voltage value of the analog signal provided as input, and to keep the voltage value stable for the duration of the conversion, before the conversion of the voltage values into binary values.
It is in particular known to produce a sample-and-hold circuit with double switching, comprising two track-and-hold modules connected in series, a first master track-and-hold module being connected in series at the output of an input amplifier with two switches, and a second slave track-and-hold module being connected in series at the output of the first track-and-hold module. The second track-and-hold module is connected at the input of an output amplifier with two switches. The second track-and-hold module is commanded by a second clock signal in phase opposition relative to the first clock signal so as to track and maintain a held sample. Each track-and-hold module is able to operate alternately according to an on mode, then a held mode, based on the command clock signal.
Such a traditional structure makes it possible to obtain, as output, an output signal that is sampled twice as quickly as the signal coming from a single track-and-hold module.
However, such a circuit with two track-and-hold modules connected in series has several drawbacks.
A first drawback is that the second track-and-hold module holds the signal coming from the first track-and-hold module at the moment of the transition between the on mode and the held mode, which creates additional switching errors, and as a result, the linearity of the sample-and-hold device is deteriorated. Linearity of the sample-and-hold (S/H) device refers to the total harmonic distortion level, which is defined as the ratio of the overall root-mean-square value of the harmonics (i.e., their quadratic sum) to the root-mean-square value of the component.
A second drawback of such a sample-and-hold circuit is that the sampling moment between the two track-and-hold modules is linked and not controllable.
Lastly, a third drawback is that the noise factor is increased, since the noises respectively introduced by the track-and-hold devices are cumulative, due to the operation in master-slave mode.